Lets say we have the reaction
$$\ce{H2 + O2 -> H2O}$$
And we are given 10 moles of $\ce{O2}$ and are asked that How many moles of hydrogen gas do we need to complete the reaction ?
Stoichiometry gives the answer as 10 moles but by applying Principle Of Atom Conservation, we get
$$\text{Moles of }\ce{H2} =2\cdot\text{moles of }\ce{O2}$$
Why is the POAC giving wrong results?